1. Field of the Invention
The present invention relates to mobile communications. More particularly, the present invention relates to a transmission control method for a Hybrid Automatic Repeat reQuest (HARQ) in a mobile communication system.
2. Description of the Related Art
Universal Mobile Telecommunications System (UMTS) is a third generation (3G) mobile telecommunication technology that evolved from Global System for Mobile communications (GSM) and General Packet Radio Services (GPRS) and uses Wideband Code Division Multiple Access (WCDMA).
The 3G Partnership Project (3GPP), which is responsible for the standardization of UMTS, is working to significantly extend the performance of UMTS in the work item Long Term Evolution (LTE). LTE is a 3GPP standard that provides for a downlink speed of up to 100 and is expected to be commercially launched in 2010. In order to fulfill the requirements for the LTE systems, studies have been done in various aspects including minimization of the number of involved nodes in the connections and placing radio protocol as close as to the radio channels.
FIG. 1 is a schematic diagram illustrating an LTE mobile communication system according to the related art.
As shown in FIG. 1, the LTE mobile communication is characterized in that the Evolved Radio Access Network (hereinafter called E-RAN) 110 and 112 has only two infrastructure nodes: the Evolved Node B (hereinafter called ENB or Node B) 120, 122, 124, 126, and 128 and the Access Gateway (hereinafter called (AG) 130 and 132. A User Equipment (hereinafter called UE) 101 accesses the Internet Protocol (IP) network 114 via the E-RAN 110 and 112.
The ENB's 120, 122, 124, 126, and 128 correspond to conventional Node B which provides the UE 101 with radio access service. However, the ENB's 120, 122, 124, 126, and 128 are responsible for more complex functions than that of the conventional Node B. In the next generation mobile communication system, all user traffic, including real time service such as Voice over IP (VoIP), is served through a shared channel. For this reason, there is a need for a device to collect status information of the UE's and schedule based on the status information. Each of the ENBs 120, 122, 124, 126, and 128 is responsible for scheduling the UE's. In order to achieve the speed of 100 Mbps or faster, the mobile communication system exploits the radio access technology of Orthogonal Frequency Division Multiplexing (OFDM) using a 20 MHz bandwidth. Also, an Adaptive Modulation and Coding (AMC) technique is supported for determining a modulation scheme and a channel coding rate according to the channel status of the UE 101.
FIG. 2 is a diagram illustrating a user plane protocol stack architecture of an LTE mobile communication system according to the related art.
As shown in FIG. 2, the UE has a protocol stack including a Packet Data Convergence Protocol (PDCP) layer 205, a Radio Link Control (RLC) layer 210, a Media Access Control (MAC) layer 215, and a Physical (PHY) layer 220. Similarly, the ENB has a protocol stack including a PDCP layer 240, an RLC layer 235, a MAC layer 230, and a PHY layer 225. The PDCP layers 205 and 240 are responsible for IP header compression/decompression. The RLC layers 210 and 235 pack the PDCP Packet Data Units (PDUs) into a size appropriate for transmission (hereinafter the data unit delivered from an upper layer entity is called PDU) and perform an Automatic Repeat reQuest (ARQ) function. The MAC layers 215 and 230 serve multiple RLC layer entities. Also, the MAC layers 215 and 230 can multiplex the RLC PDUs produced by the RLC layer entities into a single MAC PDU and de-multiplex a MAC PDU into the RLC PDUs. The physical layers 220 and 225 perform encoding and modulation on the upper layer data to transmit through a radio channel and perform demodulation and decoding on the OFDM symbols received through radio channel for delivery to upper layers.
In the LTE mobile communication system, a Hybrid Automatic Repeat reQuest (HARQ) function is provided for more reliable transmission of uplink MAC PDUs. In the mobile communication system using HARQ, when failing receipt of a MAC PDU, the receiver transmits an HARQ Not Acknowledgement (NACK) to the transmitter such that the transmitter, after receiving the HARQ NACK, retransmits the MAC PDU. The receiver performs soft combining to combine the initial transmission and retransmissions.
FIG. 3 is a diagram illustrating a message format of a resource assignment message according to the related art. In the LTE mobile communication system, downlink resource assignment is performed by transmitting a resource assignment message.
Referring to FIG. 3, the resource assignment message includes a Resource Block (RB) assignment field 305 carrying information on the amount and position of the transmission resource. In the LTE mobile communication system, the resource is assigned in units of resource block defined as a frequency bandwidth in length of 1 msec, and the assigned resource is indicated by the RB assignment field 305. The resource assignment message also includes a Modulation and Coding Scheme (MCS) field 310 which indicates the adaptive modulation and coding formats for the transmission data. The resource assignment message also includes a New Data Indicator (NDI) 315. The NDI 315 is 1-bit information to indicate whether the transmission resource assignment message is of either an initial transmission or retransmission. The resource assignment message also includes several other fields designated by reference 330 which are not of concern to the present invention. Accordingly and for sake of brevity, those other fields are not discussed herein.
In conventional downlink data transmission, the base station transmits the resource assignment message using a Physical Downlink Control Channel (PDCCH), and the resource assignment message includes a Cyclic Redundancy Check (CRC) 335. The CRC 335 is computed based on the payload of the resource assignment message and Cell-Radio Network Temporary Identifier (C-RNTI) as a temporary User Equipment (UE) ID. That is, the base station performs the CRC calculation on the data including the payload of the resource assignment message and the C-RNTI, and the CRC calculation result is attached to the resource assignment message. The UE performs CRC testing on the resource assignment message received through the PDCCH with its C-RNTI. If the CRC test is passed, it is determined that the resource assignment message is destined to the UE, whereby the UE locates the resource indicated by the RB assignment field 305 and receives the downlink data transmitted on the resource. The downlink data is transmitted on a Physical Downlink Shared Channel (PDSCH).
Since the resource for the HARQ retransmission is also assigned by means of the resource assignment message, the LTE system is configured to transmit multiple resource assignment messages for a single packet. In a service that periodically generates small packets at a relatively short interval, such as Voice over IP (VoIP) service, it is inefficient to transmit the resource assignment message in a per-packet manner. In order to mitigate this inefficiency, a Semi-Persistent Scheduling (SPS) has been introduced. In the SPS, the resource assigned to the UE is maintained and thus no transmission of additional resource assignment message is required. In more detail, the base station assigns the resource to the UE by means of an SPS resource assignment message on the PDCCH and transmits the packets on the SPS resource.
The SPS resource assignment message may have the same format as the normal resource assignment message as shown in FIG. 3. In order to discriminate between the normal resource assignment message and the SPS resource assignment message, the UE is assigned a separate C-RNTI called SPS C-RNTI. That is, the UE is assigned both the normal C-RNTI and the SPS C-RNTI and performs the CRC test with both the C-RNTI and the SPS C-RNTI. If the CRC test is passed with the normal C-RNTI, this means that the resource assignment message is the normal resource assignment message and otherwise, if the CRC test is passed with the SPS C-RNTI, this means that the resource assignment message is the SPS resource assignment message. In case that the CRC test fails with both the normal C-RNTI and the SPS C-RNTI, the resource assignment message is not destined to the UE.
Once the SPS resource is assigned to the UE, the UE receives the periodically repeating data on the SPS resource. In downlink, only initial transmissions of the HARQ are transmitted on the SPS resource. If the CRC test fails for the packet received on the SPS resource, the UE transmits an HARQ NACK to the base station and, the base station performs the HARQ retransmission using the normal resource assignment. At this time, the retransmission of the packet received on the SPS resource is identified by the SPS C-RNTI. In this case, the NDI is used to discriminate between the initial transmission and retransmission of the received packet. However, since the initial transmission of a packet on the SPS resource is performed without a resource assignment message, it is difficult to discriminate between the initial transmission and retransmission based on toggling of the NDI. Accordingly, when the SPS C-RNTI is used, the meaning of the NDI is fixed to discriminate between the initial transmission and retransmission. For instance, a value of the NDI is set to 1 for retransmission and set to 0 for SPS resource assignment.
Since the NDI is used per HARQ process, when the downlink data transmitted with the normal C-RNTI and the SPS C-RNTI are handled in the same HARQ process, there is no way to distinguish between the initial transmission and retransmission based on the NDI.